Iraqi special forces Brig. Gen. Haider Fadhil said French officials tested the chemical agent this week and confirmed it was sulfur mustard.

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Iraqi forces showed journalists a tank of the chemical agent and a warehouse of more than a dozen surface-to-surface rockets bearing Russian inscriptions in eastern Mosul.

Fadhil said the types of rockets found lead him to suspect the Islamic State group was experimenting with the rockets to develop a way to weaponize the chemical agent.

Iraqi and US officials have repeatedly warned of IS efforts to develop chemical weapons. The extremist group has launched attacks using low-grade chemical weapons in Iraq, causing a handful of casualties.